FIG. 1 shows an image based on the opaque ink model. The image is composed of two objects, the characters "A" and "B", having two different colors; "A" may be blue, and "B" may be red. In this example the character "B" is said to be superior to the character "A" (and conversely, the character "A" is said to be inferior to "B"), because the red of "B" is printed instead of the blue ink of "A" where the two objects overlap. Thus, in the final image it appears that the ink making up the object "B" is opaque (i.e., it does not blend with any ink that may be underlying it) and that the ink of "B" overlays the ink of the object "A", so that the ink of "A" cannot be seen.
If the characters are a shade of grey or have a color other than black, they are halftone screened, a process well known in the art, which uses dots of different sizes in order to create the image. In multicolor work typically three colored inks in addition to black ink are used in order to create the desired colors. The halftone dots of each color are typically arranged in a grid, which is referred to in the art as a screen. In order to avoid moire effects, these screens are rotated with respect to each other. Means for generating these rotated screens are well known in the art.
In a typical prior art system for interpreting image information, tone values, which represent the shade of grey or the intensity of a color and which are typically stored in a display list, are screened 21, as shown in FIG. 2. The operator chooses a screen with respect to which the tone values are to be screened, e.g., a grid rotated 15.degree. and having a dot frequency of 150 dots per inch. The output of this screening, which is typically done in software, is a long string of bits, indicating whether or not ink should be applied at a certain point, known as a pixel, in the image. The next step 22 in the prior art system shown in FIG. 2 is the insertion of these bits into a bit map. For images based on the opaque ink model, bits representing inferior objects are inserted into the bit map first, so that bits representing a superior object will overlay the bits representing an inferior object, thereby overwriting the inferior bits. After all the bits have been read into the bit map, the bit map may then be used to control the plotter. Because this prior art system performs most of the processing of the image information at the bit level, a good deal of processing must be performed during screening and plotting, especially when the output has high resolution.